1. Field of the Invention
The present invention relates to a combustor in which a liquid fuel is preevaporated, premixed with a combustion air and burned, and to a combustion apparatus comprising the combustor and a combusting process.
2. Description of the Related Art
In order to increase the loading of a gas turbine combustor, there has been a recent tendency to use a preevaporated/premixed type combustion wherein a liquid fuel is vaporized (preevaporated) and premixed with air, and the resulting premixture (premixed gas) is ejected from the same nozzle. The use of such preevaporated/premixed combustion is advantageous in the following two respects. One is that the use of the pre-evaporating and pre-mixing type combustion enables the reaction region for combustion to be reduced in size. In other words, the flame can be shortened to enable a high load combustion. The other advantage is that the amount of NOx discharged can be reduced by employing a dilute fuel mixture in the combustion process.
Another type of combustion different from the premixing type combustion includes a diffusion combustion wherein air and a fuel are ejected from different nozzles. In diffusion combustion, a region in which the ratio of a theoretical air amount to an actual air amount in a fuel-air mixture (which will be referred to as an air ratio) is of about 1:1 is of always present in the course of the premixing of a fuel with air in a combustion chamber, even if combustion is conducted under a dilute-fuel condition. The temperature of a flame is higher in the vicinity of the region with an air ratio of about 1:1 and for this reason, a reduction in the amount of NOx is generally difficult.
On the other hand, in a premixing type combustion of a higher air ratio, i.e., in a premixing type combustion wherein an excess amount of air and a fuel are premixed and the resulting mixture is burned, the fuel-air mixture is burned under such a combustion condition that the fuel is dilute in the entire combustion region and hence, a reduction in the amount of NOx is easy. Such a dilute premixing combustion process is being increasingly used in a combustor for a gas turbine (for example, see Japanese Patent Publication No. 35016/87).
In general, to realize a premixing combustion using a liquid fuel, a premixing type combustion burner is provided which includes an atomizer for atomizing the liquid fuel, and an evaporation chamber in which the atomized liquid fuel is evaporated (see Gas Turbine Academic Society Journal, Vol 16, No. 64, PP 47 to 55).
In such a premixing type combustion burner, there are requirements for the formation of finely-divided atomized particles intended to hasten the evaporation of the fuel, and a formation of a uniform preevaporated-fuel/air premixture resulting from the mixing of the atomized particles with air.
An example of a premixing type burner which is at least capable of the formation of finely-divided atomized particles among the requirements included for a premixing type burner, is shown in FIG. 10 (see Prog. Energy Combust. Sci., No. 6, pp 233 to 261).
This premixing type burner comprises an inner cylinder. A liquid fuel is supplied to an inner peripheral surface of the inner cylinder. A pintle 5 is mounted within the inner cylinder 2. The diameter of the pintle is increased in diameter in a downstream direction. A prefilming surface 3 is formed around an inner periphery of the inner cylinder at its downstream side and gradually increased in diameter in the downstream direction.
Air running straightly through the inner cylinder 2 serves to form the liquid fuel into a film-like configuration to guide it to an atomizing lip 4. The film-like liquid fuel which has reached the atomizing lip 4 is ejected from the atomizing lip 4 toward an outer periphery by the air passed through the inner cylinder 2, and is sheared and finely divided by the air running straightly along the outside of the inner cylinder 2.
The finely dividing capability and the spray dispersing property are dependent upon the shape of the pintle 5. This is because the angle of ejection of the liquid fuel with respect to the air running straightly along the outside of the inner cylinder 2 is determined by the shape of the pintle 5.